Spectroscopy of the local density of states in nanowires using integrated quantum dots

In quantum dot (QD) electron transport experiments, additional features can appear in the differential conductance dI/dV that do not originate from discrete states in the QD, but rather from a modulation of the density of states (DOS) in the leads. These features are particularly pronounced when the leads are strongly confined low-dimensional systems, such as in a nanowire (NW) where transport is one dimensional and quasi-zero-dimensional lead states can emerge. We study such lead states in InAs NWs. We use a QD integrated directly into the NW during the epitaxial growth as an energetically and spatially well-defined tunnel probe to perform dI/dV spectroscopy of discrete bound states in the "left"and "right"NW lead segments. By tuning a sidegate in close proximity of one lead segment, we can distinguish transport features related to the modulation in the lead DOS and to excited states in the QD. We implement a noninteracting capacitance model and derive expressions for the slopes of QD and lead resonances that appear in two-dimensional plots of dI/dV as a function of source-drain bias and gate voltage in terms of the different lever arms determined by the capacitive couplings. We discuss how the interplay between the lever arms affects the slopes. We verify our model by numerically calculating the dI/dV using a resonant tunneling model with three noninteracting quantum dots in series. Finally, we use the model to describe the measured dI/dV spectra and quantitatively extract the tunnel couplings of the lead segments. Our results constitute an important step towards a quantitative understanding of normal and superconducting subgap states in hybrid NW devices.